Methods and apparatus for multi-carrier communication systems with adaptive transmission and feedback

ABSTRACT

An arrangement is disclosed where in a multi-carrier communication system, the modulation scheme, coding attributes, training pilots, and signal power may be adjusted to adapt to channel conditions in order to maximize the overall system capacity and spectral efficiency without wasting radio resources or compromising error probability performance, etc.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No.17/094,286, filed Nov. 10, 2020, which is a continuation of U.S. patentapplication Ser. No. 16/126,896, filed Sep. 10, 2018, which issued asU.S. Pat. No. 10,834,706 on Nov. 10, 2020, which is a continuation ofU.S. patent application Ser. No. 15/082,878, filed Mar. 28, 2016, whichissued as U.S. Pat. No. 10,075,941 on Sep. 11, 2018, which is acontinuation of U.S. patent application Ser. No. 14/539,917, filed Nov.12, 2014, which issued as U.S. Pat. No. 9,301,296 on Mar. 29, 2016,which is a continuation of U.S. patent application Ser. No. 13/246,677,filed on Sep. 27, 2011, which is a continuation of U.S. patentapplication Ser. No. 12/755,313, filed on Apr. 6, 2010, which issued asU.S. Pat. No. 8,027,367 on Sep. 27, 2011, which is a continuation ofU.S. patent application Ser. No. 10/583,529, filed on May 10, 2007,which issued as U.S. Pat. No. 7,693,032 on Apr. 6, 2010, which is aNational Stage Entry of PCT/US2005/004601, filed on Feb. 14, 2005, whichclaims the benefit of U.S. Provisional Application No. 60/544,521, filedon Feb. 13, 2004, which is/are incorporated by reference as if fully setforth. This application also relates to PCT Application No.PCT/US05/03518 titled “Methods and Apparatus for OverlayingMulti-Carrier and Direct Sequence Spread Spectrum Signals in a BroadbandWireless Communication System,” filed Jan. 27, 2005, which claims thebenefit of U.S. Provisional Application No. 60/540,032 filed Jan. 29,2004 and U.S. Provisional Application No. 60/540,586 filed Jan. 30,2004, the contents of which are incorporated herein by reference.

BACKGROUND

Adaptive modulation and coding (AMC) has been used in wireless systemsto improve spectral efficiency in a fading environment where signalquality varies significantly. By adjusting the modulation and codingscheme (MCS) in accordance with the varyingsignal-to-interference-plus-noise ratio (SINR), reliable communicationlink can be maintained between communicating devices. For example, inCDMA2000 1×EV-DO system, twelve different modulation/coding schemes areprovided. AMC is also used in CDMA2000 1×EV-DV and 3GPP HSDPA systems.

To improve performance, in addition to the MCS, other system functionssuch as channel estimation, transmission power control (TPC), andsubchannel configuration can be adjusted in accordance with the state ofthe communication channel. For example, channel estimation typicallyutilizes training symbols or pilot data, which are known to both thetransmitter and the receiver. For coherent modulation, the channelinformation can be extracted at the receiver by comparing the pilots andtheir corresponding received versions. For non-coherent modulation, thereceived samples of the pilots are used as reference for the detectionof the transmitted data.

Channel estimation is an important part of multi-carrier (MC)communication systems such as Orthogonal Frequency Division Multiplexing(OFDM) systems. In conventional OFDM systems, such as IEEE802.11a,802.11g, 802.16, or DVB-T system, pilots are transmitted for channelestimation. The pilots are fixed and form part of other functions suchas MCS, TPC, and subchannel configuration in some wireless systems.

Fast TPC can compensate for fast fading. In a multi-cell multiple-accesssystem, TPC is also used to reduce intra-cell and inter-cellinterference and to conserve battery life for the mobile station bytransmitting with only necessary power. TPC is one of many functions insome wireless systems, along with MCS, pilot attributes, subchannelconfiguration, etc.

The subchannel configuration is normally defined and fixed in anoperation, and it is usually not considered an adjustable function ofthe system to be adapted to the user profile and/or operationalenvironment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a representative cellular communication system.

FIG. 2 is a basic structure of a multi-carrier signal in the frequencydomain, made up of subcarriers.

FIG. 3 depicts a radio resource divided into small units in bothfrequency and time domains: subchannels and time slots.

FIG. 4 is an illustration of a control process between Device A andDevice B, each of which can be a part of a base station and a mobilestation depicted in FIG. 1.

FIG. 5 illustrates a joint adaptation process at a transmitter of anOFDM system which controls coding, modulation, training pilot pattern,and transmission power for a subchannel.

FIG. 6 is an illustration of a control messaging associated with datatransmission between communication devices.

FIG. 7 illustrates two different training pilot patterns plotted for amulti-carrier system.

FIG. 8 illustrates a power control in AMCTP scheme for an OFDM system.

DETAILED DESCRIPTION

Methods and apparatus for adaptive transmission of wirelesscommunication signals are described, where MCS (modulation and codingscheme), coding rates, training pilot patterns, TPC (transmission powercontrol) levels, and subchannel configurations are jointly adjusted toadapt to the channel conditions. This adaptation maximizes the overallsystem capacity and spectral efficiency without wasting radio resourcesor compromising error probability performance.

Furthermore, the subchannel composition is designed to be configurableso that it can be adjusted statically or dynamically according to theuser profiles or environmental conditions. The methods for obtaining thechannel information and for transmitting the control information in thejoint adaptation scheme are also described below, such as feedback ofchannel condition and indexing of the joint scheme, along with methodsfor reducing the overhead of messaging.

The mentioned multi-carrier system can be of any special format such asOFDM, or Multi-Carrier Code Division Multiple Access (MC-CDMA) and canbe applied to downlink, uplink, or both, where the duplexing techniqueis either Time Division Duplexing (TDD) or Frequency Division Duplexing(FDD).

The apparatus and methods are described with respect to variousembodiments and provide specific details for a thorough understandingand enablement. One skilled in the art will understand that theinvention may be practiced without such details. In some instanceswell-known structures and functions are not shown or described in detailto avoid unnecessarily obscuring the description of the embodiments.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise,” “comprising,” and thelike are to be construed in an inclusive sense as opposed to anexclusive or exhaustive sense; that is to say, in the sense of“including, but not limited to.” Words using the singular or pluralnumber also include the plural or singular number respectively.Additionally, the words “herein,” “above,” “below” and words of similarimport, when used in this application, shall refer to this applicationas a whole and not to any particular portions of this application. Whenthe claims use the word “or” in reference to a list of two or moreitems, that word covers all of the following interpretations of theword: any of the items in the list, all of the items in the list and anycombination of the items in the list.

The content of this description is applicable to a communication systemwith multiple transmitters and multiple receivers. For example, in awireless network, there are a number of base stations, each of whichprovides coverage to its designated area, typically called a cell.Within each cell, there are mobile stations. FIG. 1 illustrates acommunication system that is representative of such a system, where BaseStation 110 is communicating with Mobile Stations 101 and 102 in SectorA of its cell site while Base Station 120 is communicating with MobileStations 103, 104, and 105 in Sector B of its cell site.

A multi-carrier multiple-access system is a special case of generalcommunication systems and hereinafter is employed as a representativecommunication system to describe the embodiments of the invention.

Multi-Carrier Communication System

The physical media resource (e.g., radio or cable) in a multi-carriercommunication system can be divided in both the frequency and the timedomains. This canonical division provides a high flexibility and finegranularity for resource sharing.

The basic structure of a multi-carrier signal in the frequency domain ismade up of subcarriers. Within a particular spectral band or channel,there are a fixed number of subcarriers, and there are three types ofsubcarriers:

-   -   1. Data subcarriers, which carry information data;    -   2. Pilot subcarriers, whose phases and amplitudes are        predetermined and made known to all receivers and which are used        for assisting system functions such as estimation of system        parameters; and    -   3. Silent subcarriers, which have no energy and are used for        guard bands and DC carrier.

The data subcarriers can be arranged into groups called subchannels tosupport scalability and multiple-access. The carriers forming onesubchannel are not necessarily adjacent to each other. Each user may usepart or all of the subchannels. The concept is illustrated in FIG. 2,which is the basic structure of a multi-carrier signal in the frequencydomain, made up of subcarriers. Data subcarriers can be grouped intosubchannels in a specified manner. The pilot subcarriers are alsodistributed over the entire channel in a specified manner.

The basic structure of a multi-carrier signal in the time domain is madeup of time slots to support multiple-access. The resource division inboth the frequency and time domains is depicted in FIG. 3, which is theradio resource divided into small units in both the frequency and timedomains (subchannels and time slots). The basic structure of amulti-carrier signal in the time domain is made up of time slots.

Adaptive Transmission and Feedback

The underlying principles of adaptive transmission and feedback are bothto increase the degree of freedom of a transmission process and tosupply information for the adaptation process of a communication system.The adaptation process adjusts the allocated modulation schemes, codingrates, pilot patterns, power levels, spatial processing schemes,subchannel configurations, etc. in accordance with the transmissionchannel state and condition, for improving system performance and/orcapacity.

Below, AMCTP (adaptive modulation, coding, training and power control)is used as a general term, where its variations can be applied toappropriate applications. There are different adaptive transmissionschemes that are subsets of the AMCTP scheme, such as AMCT (adaptivemodulation, coding and training), AMTP (adaptive modulation, training,and power control), AMT (adaptive modulation and training), and soforth.

FIG. 4 is an illustration of the control process between Device A andDevice B, each of which can be a part of a base station and a mobilestation depicted in FIG. 1, during adaptive transmission. Thetransmitter 401 of Device A transmits data 402 and associated controlinformation 404 to Device B, based on an output of the adaptationprocess 406. After a receiver 408 of Device B receives the transmitteddata 402 and control information 404, a measurement process 410 ofDevice B measures a channel conditions and feeds a channel qualityinformation (CQI) 412 back to Device A.

The granularity of AMCTP schemes in a multi-carrier system can beuser-based, where one or multiple subchannels may be used, or thegranularity can be subchannel-based, where a subchannel may contain oneor more subcarriers. Likewise, the granularity of CQI can be user- orsubchannel-based. Both AMCTP and CQI may change over time and may differfrom one time slot to another.

FIG. 5 illustrates a joint adaptation process at a transmitter of anOFDM system which employs separate processing block to control thecoding 502, modulation 504, training pilot pattern 506, and transmissionpower for a subchannel 508. Each block may be implemented combined orseparately in circuitry, in dedicated processors, in a digital signalprocessor, as a microprocessor implemented subroutine, etc.

FIG. 6 is an illustration of control messaging associated with the datatransmission between communication devices, such as Device A and B inFIG. 4. In FIG. 6 the AMCTP indicator 602 is associated with datatransmission 604 on a forward link from the transmitter to the receiver,and CQI 606 is associated with the information feedback from thereceiver to the transmitter on a return channel.

In a system where AMCTP is used, the transmitter relies on the CQI toselect an appropriate AMCTP scheme for transmitting the next packet, orretransmitting a previously failed packet, required in an automaticrepeat request (ARQ) process. The CQI is a function of one or more ofthe following: received signal strength; average SINR; variance in time;frequency or space; measured bit error rate (BER); frame error rate(FER); or mean square error (MSE). Channel conditions hereinafter arereferred to as one or more of the following, for a user or a subchannel:signal level, noise level, interference level, SINR, fading channelcharacteristics (Doppler frequency, delay spread, etc.), or channelprofile in time or frequency domain. The detection of the channelcondition can be at the transmitter, the receiver, or both.

An MCS in AMCTP is referred to as a modulation and error correctioncoding scheme used in the system. By matching an MCS to a specificchannel condition (e.g., SINR level), a better throughput is achieved.Varying only the MCS is a sub-optimal approach since other factors suchas training pilot patterns or subchannel compositions also impact systemperformance.

A pilot pattern includes the number of (training) pilot symbols, thelocation of the symbols in time/frequency/space, the amplitude andphase, and other attributes of these symbols. The system may usedistinctive pilot patterns to suit different MCS and channel conditions.The pilot pattern requirements for a robust channel estimation vary withthe SINR of the channel and the channel profile.

In a multi-carrier system, pilots are transmitted on certain positionsin the time-frequency grid. FIG. 7 illustrates two of many differenttraining pilot patterns that may be used, each plotted for amulti-carrier system, where the dark shaded time-frequency grids 702 areallocated as training pilot symbols. One criterion for choosing a pilotpattern is that the pilot assisted channel estimation should not be abottleneck for the link performance, and that the pilot overhead shouldbe kept to a minimum. The joined adaptation of training pilot patterntogether with MCS is a more effective way of matching the channelconditions, and results in a better performance compared with a mereadaptation of MCS.

The power control information may include an absolute power level and/ora relative amount to increase or decrease the current power setting. Ina multi-carrier system, the power levels of different subchannels areset differently such that minimum power is allocated to a subchannel tosatisfy its performance requirements while minimizing interference toother users.

The power control can be user- or subchannel-based. FIG. 8 is anillustration of a power control in an OFDM system where digital variablegains 802 G1, G2 . . . GN are applied to subchannels 804 that may havedifferent MCSs with different transmission power levels. Analog domaingain 806 Ga is used to control the total transmission power signalprocesses to meet the requirements of the transmission power of thedevice. In FIG. 8, after variable gains are applied to subchannels 804,they are inputted to the inverse discrete Fourier transform (IDFT)module. The outputs from the IDFT are the time domain signals, which areconverted from parallel to sequential signals after a cyclic prefix isadded to them.

Table 1 is an example of a general AMCTP table (or CQI table). It shouldbe noted that some pilot patterns in the table can be the same. Thetotal number of indexes used to represent different combinations of thejoint adaptation process can be different for AMCTP index and CQI index.For instance, it is not necessary to send absolute transmission powerinformation to the receiver(s). Some AMCTP information, such as relativepower control or code rate, can be embedded in the data transmissioninstead of being conveyed in the AMCTP index.

TABLE 1 An example of general AMCTP. Code Training Transmit IndexModulation Rate Pilot Power 1 QPSK 1/16 Pattern 1 + 2 QPSK ⅛ Pattern 2 +3 QPSK ¼ Pattern 3 + 4 QPSK ½ Pattern 4 + 5 QPSK ½ Pattern 5 + 6 16QAM ½Pattern 6 + 7 16QAM ½ Pattern 7 + 8 16QAM ¾ Pattern 8 + 9 16QAM ¾Pattern 9 + 10 64QAM ⅔ Pattern 10 + 11 64QAM ⅚ Pattern 11 + 12 64QAM ⅚Pattern 12 Max-1x 13 64QAM ⅚ Pattern 13 Max-2x 14 64QAM ⅚ Pattern 14Max-3x

In a general AMCTP or CQI table, different training pilot patterns maybe used for different modulations and code rates. However, even for thesame modulation and coding, different patterns can be used to matchdifferent channel conditions. In order to make the table more efficient,more indexes can be allocated to the more frequently used scenarios. Forexample, several training pilot patterns can be allocated to the sameMCS that is used more frequently, to achieve finer granularity and thushave a better match with different channel conditions.

Table 2 is a simple realization of the AMCTP index or the CQI index. Inone embodiment, as shown in Table 2, the AMCTP and CQI index is Graycoded so that one bit error in the index makes the index shift to theadjacent index.

In some cases, a different number of pilot symbols is used for the sameMCS. In one embodiment, to keep the packet size the same when the sameMCS is used with a different number of pilot symbols, rate matchingschemes such as repetition or puncturing is employed. For instance inTable 2, for Index 010 and Index 011, Pattern 3 has more pilot symbolscompared to Pattern 2. The code rate of Index 010 is 1/2, which ispunctured to 7/16 for Index 011 to accommodate the extra pilot symbols.In one embodiment, more significant bits in the CQI index are protectedwith stronger error protection code on the return channel.

TABLE 2 Another example of AMCTP or CQI table. Index (Gray Code TrainingTransmit coded) Modulation Rate Pilot Power 000 QPSK ¼ Pattern 1 Max 010QPSK ½ Pattern 2 Max 011 QPSK 7/16 Pattern 3 Max 001 16QAM ½ Pattern 2Max 101 16QAM 7/16 Pattern 3 Max 111 64QAM ⅔ Pattern 2 Max 110 64QAM ⅚Pattern 3 Max 100 64QAM ⅚ Pattern 3 Max-X

Other factors that can be used in the adaptation process includemodulation constellation arrangements, transmitter antenna techniques,and subchannel configuration in a multi-carrier system.

For some modulation schemes such as 16QAM and 64QAM, how informationbits are mapped to a symbol determines the modulation schemes'reliability. In one embodiment, constellation arrangement is adjusted inthe adaptation process to achieve a better system performance,especially during retransmission in a hybrid ARQ process.

Some multiple antenna techniques, such as transmission diversity, areused to improve the transmission robustness against fading channeleffects, whereas other multiple antenna techniques such asmultiple-input multiple-output (MIMO) schemes are used to improvetransmission throughput in favorable channel conditions. In oneembodiment of the adaptive transmissions the antenna technique used fora transmission is determined by the adaptation process.

In a multi-carrier multi-cell communication system, when all subcarriersin one subchannel are adjacent or close to each other, they are morelikely to fall in the coherent bandwidth of a fading channel; thus theycan be allocated to users that are either fixed in location or are moveslowly. On the other hand, when subcarriers and/or subchannels thatbelong to one user are scattered in the frequency domain, it results inhigher diversity gains for the fast moving users, and a betterinterference averaging effect.

Given the fact that different configurations of subchannel compositionsare suitable for different scenarios or user profiles, subchannelconfiguration is included in the transmission adaptation process. In oneembodiment, the subchannel configuration information is broadcast on thecommon forward control channel to all users such that each user isinformed of its subchannel configuration.

In another embodiment, the subchannel configuration is adjustedaccording to deployment scenarios. For instance, when a base station isnewly deployed with less interference, one form of subchannelconfiguration is used, and when more users join the network or moreadjacent base stations are deployed, which results in strongerinterference to the users in the system, a different subchannelconfiguration with better interference averaging effect is used.

The following paragraphs describe a method of transmitting the controlmessage between the transmitter and receiver, when the AMCTP scheme isimplemented. A forward control link is defined here as the transmissionof the AMCTP indicator from the transmitter to the receiver, and areturn control channel is defined as the transmission of CQI, as thefeedback information, from the receiver to the transmitter, as shown inFIG. 4.

The AMCTP indicator on the forward link can be sent either separately orjointly. For instance, the power control information, training pilotpattern indicator, or antenna diversity scheme can be embedded in thedata transmission. In another embodiment, AMCTP is transmitted on aseparate control channel with stronger error protection.

One way for the transmitter to obtain CQI is to have it explicitly sentfrom the receiver to the transmitter based on channel conditionmeasurements at the receiver during previous transmission(s). The CQI isthen used by the transmitter to determine what AMCTP scheme to use forthe next transmission. In one embodiment, CQI for one user isperiodically updated on the return channel, even when there is noforward transmission targeted for that user. In this case the receivermeasures the channel conditions from the common broadcast transmissionor the data transmission targeted to other users.

In one embodiment, the transmitter or the receiver uses any of severalknown predictive algorithms to predict current or future channelconditions based on previous channel measurements. This is moreeffective for a fast fading environment where the past measurements maynot match the current transmission closely, due to the fast channelvariations. The output of the predictive algorithm is then used by theadaptation process to select the best possible scheme for the currenttransmission.

Another method to obtain CQI is through the transmission of a probingsequence from the receiver to the transmitter on the return channel. Inone embodiment, in a multi-carrier communication system, a probingsequence is transmitted from the receiver to the transmitter using anoverlay scheme where the probing sequence is overlaid to the datatraffic without having negative impact on the data transmissionperformance. In this case the transmitter estimates the channel profilein the time and/or frequency domains based on the received probingsequence. This is especially effective for TDD systems due to thereciprocity of the channel conditions on forward and reverse channels.

The AMCTP indicator or CQI can be sent per user or per subchannel. Inone embodiment if per subchannel feedback is employed, since the AMCTPand CQI information for the same users are highly correlated, first thesource coding is employed to compress the CQI, and then the errorcorrection coding is applied to the compressed CQI to provide sufficienterror protection.

In another embodiment, in hybrid ARQ retransmission, the transmitter maynot use the requested CQI for the retransmission, while it may use therequested CQI for a new packet transmission. Instead, in thisembodiment, it selects an AMCTP scheme that is appropriate for theretransmission at the same power level as in the previoustransmission(s), in order to reduce interference with other users.

It should be pointed out that the AMCTP index used for the transmissionfrom the transmitter to the receiver may be different from the CQI thatthe receiver requested, because the transmitter may have otherconsiderations such as quality of service (QoS) for different users,network traffic load, and power allocation limit.

The above detailed description of the embodiments of the invention isnot intended to be exhaustive or to limit the invention to the preciseform disclosed above or to the particular field of usage mentioned inthis disclosure. While specific embodiments of, and examples for, theinvention are described above for illustrative purposes, variousequivalent modifications are possible within the scope of the invention,as those skilled in the relevant art will recognize. Also, the teachingsof the invention provided herein can be applied to other systems, notnecessarily the system described above. The elements and acts of thevarious embodiments described above can be combined to provide furtherembodiments.

All of the above patents and applications and other references,including any that may be listed in accompanying filing papers, areincorporated herein by reference. Aspects of the invention can bemodified, if necessary, to employ the systems, functions, and conceptsof the various references described above to provide yet furtherembodiments of the invention.

Changes can be made to the invention in light of the above “DetailedDescription.” While the above description details certain embodiments ofthe invention and describes the best mode contemplated, no matter howdetailed the above appears in text, the invention can be practiced inmany ways. Therefore, implementation details may vary considerably whilestill being encompassed by the invention disclosed herein. As notedabove, particular terminology used when describing certain features oraspects of the invention should not be taken to imply that theterminology is being redefined herein to be restricted to any specificcharacteristics, features, or aspects of the invention with which thatterminology is associated.

In general, the terms used in the following claims should not beconstrued to limit the invention to the specific embodiments disclosedin the specification, unless the above Detailed Description sectionexplicitly defines such terms. Accordingly, the actual scope of theinvention encompasses not only the disclosed embodiments, but also allequivalent ways of practicing or implementing the invention under theclaims.

While certain aspects of the invention are presented below in certainclaim forms, the inventors contemplate the various aspects of theinvention in any number of claim forms. Accordingly, the inventorsreserve the right to add additional claims after filing the applicationto pursue such additional claim forms for other aspects of theinvention.

What is claimed is:
 1. A method performed by a mobile station served bya base station using Orthogonal Frequency Division Multiplexing (OFDM)and utilizing time slots in a time domain and subchannels in a frequencydomain, the method comprising: receiving a first control messageindicating a pilot pattern out of a set of pilot patterns including afirst pilot pattern and a second pilot pattern, wherein the first pilotpattern and the second pilot pattern have pilots in a same number ofOFDM symbols, and further wherein the second pilot pattern has pilots inmore subcarriers than the first pilot pattern; transmitting channelquality information (CQI) to the base station; and in response to thetransmitted CQI and in response to the indicated pilot pattern being thefirst pilot pattern, receiving downlink data having pilots insertedusing the first pilot pattern for a first multiple-input multiple-output(MIMO) stream; and in response to the transmitted CQI and in response tothe indicated pilot pattern being the second pilot pattern, receivingdownlink data having pilots inserted using the second pilot pattern fora second MIMO stream.
 2. The method of claim 1, wherein: all pilots ofthe first pilot pattern are present in a same subset of subcarriers in asubset of OFDM symbols and no pilots of the first pilot pattern arepresent in at least another subset of OFDM symbols, wherein the subsetof OFDM symbols is different than the another subset of OFDM symbols,and a first subset of pilots of the second pilot pattern are present ina same first subset of subcarriers in a first subset of OFDM symbols,wherein a second subset of pilots of the second pilot pattern arepresent in a same second subset of subcarriers in a second subset ofOFDM symbols, wherein the first subset of subcarriers is different thanthe second subset of subcarriers, wherein the first subset of OFDMsymbols is different than the second subset of OFDM symbols, and whereinno pilots of the second pilot pattern are present in at least some OFDMsymbols.
 3. The method of claim 1, wherein: the first pilot pattern hasa set of subcarriers that have pilots, wherein a same number of pilotsare located in each subcarrier of the set of subcarriers, and whereineach pilot of the number of pilots in each subcarrier is located in asame OFDM symbol as other pilots of the number of pilots that arelocated in other subcarriers of the set of subcarriers; and the secondpilot pattern has: a first set of pilots located in a first set ofsubcarriers and a first set of OFDM symbols; and a second set of pilotslocated in a second set of subcarriers and a second set of OFDM symbols;a same first number of pilots are located in each subcarrier of thefirst set of subcarriers, and wherein each pilot of the first number ofpilots in each subcarrier of the first set of subcarriers is located ina same OFDM symbol as another pilot that is located in anothersubcarrier of the first set of subcarriers; a same second number ofpilots are located in each subcarrier of the second set of subcarriers,and wherein each pilot of the second number of pilots in each subcarrierof the second set of subcarriers is located in a same OFDM symbol asanother pilot that is located in another subcarrier of the second set ofsubcarrier; the first set of OFDM symbols are different than the secondset of OFDM symbols; and each OFDM symbol of the first set of OFDMsymbols is spaced apart from each OFDM symbol of the second set of OFDMsymbols by at least one symbol in time.
 4. The method of claim 1,wherein a location for each pilot is defined by a specific subcarrierand a specific OFDM symbol, and further wherein: the first pilot patternhas pilots located in a first set of subcarriers and a first set of OFDMsymbols, wherein the pilots of the first pilot pattern are located atall combinations of each of the first set of subcarriers with each ofthe first set of OFDM symbols; and the second pilot pattern has pilotslocated in a second set of subcarriers and a second set of OFDM symbols,wherein the pilots of the second pilot pattern are located in a subsetof all combinations of each of the second set of subcarriers with eachof the second set of OFDM symbols.
 5. The method of claim 4, wherein thefirst set of subcarriers consists of three non-adjacent subcarrierscomprising a low subcarrier, middle subcarrier, and high subcarrier,wherein the middle subcarrier is separated from the high subcarrier andthe low subcarrier by the same amount of subcarriers.
 6. The method ofclaim 5 wherein the low subcarrier is the second lowest subcarrieravailable to the first pilot pattern.
 7. The method of claim 4 whereinthe first set of OFDM symbols includes a first OFDM symbol and a secondOFDM symbol, where the first OFDM symbol and the second OFDM symbol arenon-adjacent and separated from each other by a single OFDM symbol. 8.The method of claim 4 wherein the first OFDM symbol in time of thesecond set of OFDM symbols has pilots on three non-adjacent subcarriersthat comprise a first subcarrier, a second subcarrier, and a thirdsubcarrier, wherein the second subcarrier is separated from both thefirst subcarrier and the third subcarrier by three subcarriers.
 9. Themethod of claim 8 wherein the second OFDM symbol in time of the secondset of OFDM symbols has pilots on non-adjacent subcarriers, where thenon-adjacent subcarriers of the second OFDM symbol do not overlap withthe first subcarrier, second subcarrier, or third subcarrier used forpilots in the first OFDM symbol.
 10. The method of claim 9 wherein thenon-adjacent subcarriers of the second OFDM symbol comprise a firstnon-adjacent subcarrier and a second non-adjacent subcarrier, wherein:the first non-adjacent subcarrier of the second OFDM symbol is locatedbetween the first and second subcarriers used for pilots in the firstOFDM symbol; and the second non-adjacent subcarrier of the second OFDMsymbol is located between the third and second subcarriers used forpilots in the first OFDM symbol.
 11. The method of claim 10 wherein: thefirst non-adjacent subcarrier of the second OFDM symbol is separatedfrom the first and second subcarriers used for pilots in the first OFDMsymbol by a single subcarrier; and the second non-adjacent subcarrier ofthe second OFDM symbol is separated from the third and secondsubcarriers used for pilots in the first OFDM symbol by a singlesubcarrier.
 12. The method of claim 11 wherein the third OFDM symbol intime of the second set of OFDM symbols has a pilot on the secondsubcarrier used for pilots in the first OFDM symbol.
 13. The method ofclaim 12 wherein the fourth OFDM symbol in time of the second set ofOFDM symbols has the same amount of pilots as the second OFDM symbol andthe pilots in the fourth OFDM symbol are located on the same subcarriersas the pilots in the second OFDM symbol.
 14. The method of claim 13wherein pilots of the second pilot pattern that are transmitted on thesame subcarrier are spaced apart by three or more OFDM symbols.
 15. Themethod of claim 1, wherein the pilot pattern specifies locations ofpilots on a time-frequency grid, and wherein the time-frequency gridincludes more OFDM symbols than subcarriers and the time-frequency gridincludes at least 10 OFDM symbols and at least 9 subcarriers.
 16. Amethod performed by a base station using Orthogonal Frequency DivisionMultiplexing (OFDM) and utilizing time slots in a time domain andsubchannels in a frequency domain, the base station serving a mobilestation and the method comprising: transmitting a first control messageto the mobile station indicating a pilot pattern out of a set of pilotpatterns including a first pilot pattern and a second pilot pattern,wherein the first pilot pattern and the second pilot pattern have pilotsin a same number of OFDM symbols, and further wherein the second pilotpattern has pilots in more subcarriers than the first pilot pattern;receiving channel quality information (CQI) from the mobile station; andin response to the received CQI and in response to the indicated pilotpattern being the first pilot pattern, transmitting downlink data havingpilots inserted using the first pilot pattern for a first multiple-inputmultiple-output (MIMO) stream; and in response to the received CQI andin response to the indicated pilot pattern being the second pilotpattern, transmitting downlink data having pilots inserted using thesecond pilot pattern fora second MIMO stream.
 17. The method of claim16, wherein: all pilots of the first pilot pattern are present in a samesubset of subcarriers in a subset of OFDM symbols and no pilots of thefirst pilot pattern are present in at least another subset of OFDMsymbols, wherein the subset of OFDM symbols is different than theanother subset of OFDM symbols, and a first subset of pilots of thesecond pilot pattern are present in a same first subset of subcarriersin a first subset of OFDM symbols, wherein a second subset of pilots ofthe second pilot pattern are present in a same second subset ofsubcarriers in a second subset of OFDM symbols, wherein the first subsetof subcarriers is different than the second subset of subcarriers,wherein the first subset of OFDM symbols is different than the secondsubset of OFDM symbols, and wherein no pilots of the second pilotpattern are present in at least some OFDM symbols.
 18. The method ofclaim 16, wherein: the first pilot pattern has a set of subcarriers thathave pilots, wherein a same number of pilots are located in eachsubcarrier of the set of subcarriers, and wherein each pilot of thenumber of pilots in each subcarrier is located in a same OFDM symbol asother pilots of the number of pilots that are located in othersubcarriers of the set of subcarriers; and the second pilot pattern has:a first set of pilots located in a first set of subcarriers and a firstset of OFDM symbols; and a second set of pilots located in a second setof subcarriers and a second set of OFDM symbols; a same first number ofpilots are located in each subcarrier of the first set of subcarriers,and wherein each pilot of the first number of pilots in each subcarrierof the first set of subcarriers is located in a same OFDM symbol asanother pilot that is located in another subcarrier of the first set ofsubcarriers; a same second number of pilots are located in eachsubcarrier of the second set of subcarriers, and wherein each pilot ofthe second number of pilots in each subcarrier of the second set ofsubcarriers is located in a same OFDM symbol as another pilot that islocated in another subcarrier of the second set of subcarrier; the firstset of OFDM symbols are different than the second set of OFDM symbols;and each OFDM symbol of the first set of OFDM symbols is spaced apartfrom each OFDM symbol of the second set of OFDM symbols by at least onesymbol in time.
 19. The method of claim 16, wherein a location for eachpilot is defined by a specific subcarrier and a specific OFDM symbol,and further wherein: the first pilot pattern has pilots located in afirst set of subcarriers and a first set of OFDM symbols, wherein thepilots of the first pilot pattern are located at all combinations ofeach of the first set of subcarriers with each of the first set of OFDMsymbols; and the second pilot pattern has pilots located in a second setof subcarriers and a second set of OFDM symbols, wherein the pilots ofthe second pilot pattern are located in a subset of all combinations ofeach of the second set of subcarriers with each of the second set ofOFDM symbols.
 20. The method of claim 19, wherein the first set ofsubcarriers consists of three non-adjacent subcarriers comprising a lowsubcarrier, middle subcarrier, and high subcarrier, wherein the middlesubcarrier is separated from the high subcarrier and the low subcarrierby the same amount of subcarriers.
 21. The method of claim 20 whereinthe low subcarrier is the second lowest subcarrier available to thefirst pilot pattern.
 22. The method of claim 19 wherein the first set ofOFDM symbols includes a first OFDM symbol and a second OFDM symbol,where the first OFDM symbol and the second OFDM symbol are non-adjacentand separated from each other by a single OFDM symbol.
 23. The method ofclaim 19 wherein the first OFDM symbol in time of the second set of OFDMsymbols has pilots on three non-adjacent subcarriers that comprise afirst subcarrier, a second subcarrier, and a third subcarrier, whereinthe second subcarrier is separated from both the first subcarrier andthe third subcarrier by three subcarriers.
 24. The method of claim 23wherein the second OFDM symbol in time of the second set of OFDM symbolshas pilots on non-adjacent subcarriers, where the non-adjacentsubcarriers of the second OFDM symbol do not overlap with the firstsubcarrier, second subcarrier, or third subcarrier used for pilots inthe first OFDM symbol.
 25. The method of claim 24 wherein thenon-adjacent subcarriers of the second OFDM symbol comprise a firstnon-adjacent subcarrier and a second non-adjacent subcarrier, wherein:the first non-adjacent subcarrier of the second OFDM symbol is locatedbetween the first and second subcarriers used for pilots in the firstOFDM symbol; and the second non-adjacent subcarrier of the second OFDMsymbol is located between the third and second subcarriers used forpilots in the first OFDM symbol.
 26. The method of claim 25 wherein: thefirst non-adjacent subcarrier of the second OFDM symbol is separatedfrom the first and second subcarriers used for pilots in the first OFDMsymbol by a single subcarrier; and the second non-adjacent subcarrier ofthe second OFDM symbol is separated from the third and secondsubcarriers used for pilots in the first OFDM symbol by a singlesubcarrier.
 27. The method of claim 26 wherein the third OFDM symbol intime of the second set of OFDM symbols has a pilot on the secondsubcarrier used for pilots in the first OFDM symbol.
 28. The method ofclaim 27 wherein the fourth OFDM symbol in time of the second set ofOFDM symbols has the same amount of pilots as the second OFDM symbol andthe pilots in the fourth OFDM symbol are located on the same subcarriersas the pilots in the second OFDM symbol.
 29. The method of claim 28wherein pilots of the second pilot pattern that are transmitted on thesame subcarrier are spaced apart by three or more OFDM symbols.
 30. Themethod of claim 16, wherein the pilot pattern specifies locations ofpilots on a time-frequency grid, and wherein the time-frequency gridincludes more OFDM symbols than subcarriers and the time-frequency gridincludes at least 10 OFDM symbols and at least 9 subcarriers.